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According to the Big Bang theory, about 13.8 billion years ago the whole Universe was a very small, extremely hot and dense region. From this tiny point, the whole universe expanded outwards to what exists today.

Evidence from red-shift

Astronomers have discovered that, in general, the further away a galaxy is, the more red-shifted its light is. This means that the further away the galaxies are, the faster they are moving. This is similar to an explosion, where the bits moving fastest travel furthest from the explosion. Red-shift data provides evidence that the Universe, including space itself, is expanding.

Evidence from CMBR

Astronomers have also discovered a cosmic microwave background radiation (CMBR). This comes from all directions in space and has a temperature of about -270°C. The CMBR is the remains of the thermal energy from the Big Bang, spread thinly across the whole Universe.

The discovery of red-shift in light from distant galaxies led to the development of the Big Bang theory. The discovery of the CMBR, after it had been predicted by the theory, provided very strong support for the Big Bang theory.